The MRI apparatus is a medical image diagnostic system which excites magnetic resonance in atomic nuclei within a section crossing a test subject, and obtains a tomographic image of the cross section according to nuclear magnetic resonance signals being generated. The MRI apparatus firstly conveys power generated from a radio frequency (RF) power source, to a transmit RF coil. Next, by using the transmit RF coil, the radio frequency is irradiated to the test subject so that a nuclear magnetic resonance signal of a hydrogen nucleus (1H) is generated. Next, the nuclear magnetic resonance signal thus generated is detected by a receive RF coil which is adhered to the test subject, and thereafter, a receiver acquires the signal. Finally, the signal being acquired is converted into an image and accordingly a tomographic image is obtained. In general, the RF power source and the transmit RF coil are connected via a coaxial cable, and the receiver and the receive RF coil are also connected via a coaxial cable (Hereinafter, the transmit RF coil and the receive RF coil are referred to as “RF coil” with no distinction). Therefore, in many cases, the RF coil, the coaxial cable, and the test subject are placed in such a manner as being close to one another.
The coaxial cable has a configuration that an inner conductor and a shield are arranged on an identical axis, placing an insulator (a dielectric) therebetween. Since the coaxial cable has a small signal loss and its shield is connected to a ground, it has an excellent electromagnetic shield property. Therefore, the coaxial cable is widely used as a connection cable for RF equipment such as the RF coil. However, if a large amount of power or extremely high frequency is treated, there may be cases where coupling occurs between the coaxial cable and the test subject or the like. In such a case, characteristic impedance of the coaxial cable may become uneven by location (hereinafter, such state is represented as “unbalance”). In this state, noise referred to as “common mode noise” may occur simultaneously reducing the shielding effect of the coaxial cable, and thus the cable may become susceptible to various noise.
The common mode noise represents unbalanced current which flows in aground line. In some cases, this noise may propagate through a floor or a ground surface, and come back after going along a large loop while picking up various types of noise. Therefore, equipment such as the RF coil, to which the coaxial cable is connected, is subjected to this noise. Furthermore, there may be an unexpected impact also on the coaxial cable itself, not only on the equipment being connected. If this kind of noise occur within the MRI apparatus, the coaxial cable may further become susceptible to coupling (specifically, electromagnetic radiation or electromagnetic induction), due to a structural reason. Consequently, performance of the RF coil to which the coaxial cable is connected may be degraded, and further deteriorating an image quality of the MRI. In some cases, unbalance of the characteristic impedance which occurs within the coaxial cable may cause generation of reflected electric power.
In order to the common mode noise, a balance-unbalance circuit referred to as “balun”) is used (for example, see the patent document 1 and the non-patent document 1)
FIG. 12A shows a representative example of the balun used in the MRI apparatus, the balun being a type to be inserted in the coaxial cable. As shown in the figure, the balun 20 is provided with a coaxial cable 10 forming a loop, and a capacitor 31 (hereinafter, a value of the capacitor is assumed as “C”) being connected in parallel with a shield 12 on both ends of the coaxial cable 10. FIG. 12B is a circuit diagram 21 showing the case where an inductance is represented by a lumped element (an inductor), the inductance of the coaxial cable 10 forming the loop of the balun 20 as shown in FIG. 12A. A center conductor 11 in the coaxial cable 10 forms the inductor 32 where the inductance is indicated as LS, and the shield 12 forms the inductor 33 where the inductance is indicated as LG. The inductor 33 and the capacitor 31 constitute a parallel resonance circuit 34 serially with respect to the shield 12 of the coaxial cable 10. Impedance (resistance) Z of the parallel resonance circuit 34 varies as shown in FIG. 12C depending on a frequency f of the voltage being applied. The frequency f which maximizes the impedance Z is referred to as a resonance frequency fRC of the parallel resonance circuit 34. In other words, in the case where noise (e.g., common mode noise) having the same frequency as the resonance frequency fRC of the parallel resonance circuit 34 flows in the shield 12, the highest impedance Z of the parallel resonance circuit may the noise in the most effective way.
The resonance frequency fRC of the parallel resonance circuit 34 of the balun 20 (hereinafter, referred to as resonance frequency of the balun 20) as shown in FIG. 12A and in FIG. 12B is decided by the formula (1). In general, the resonance frequency fRC being the frequency at which the balun the common mode noise is tuned to be a nuclear magnetic resonance frequency f0 of a measured nuclear species (e.g., hydrogen atomic nucleus) in the MRI apparatus.
                              f          0                =                              f            RC                    =                      1                          2              ⁢              π              ⁢                                                                    L                    G                                    ⁢                  C                                                                                        (        1        )            